1. Field of the Invention
The present invention relates to an image capture apparatus which allows for prevention of a flicker noise, more particularly to a compact image capture apparatus which is suitably combined with a mobile terminal such as an information technology equipment or a mobile phone.
2. Description of the Background Art
As generally known, an image of one subject which is captured under illumination provided using an alternating current (ac) power is liable to contain a flicker noise in an image signal thereof. One conventional solution to correct such a flicker noise is as disclosed in Japanese Patent Application Laid-Open No. 52-122039, for example. This reference discloses a method as follows. First, a waveform having a phase reverse to that of a flicker noise is obtained by synthesizing a vertical scanning frequency of an image capture apparatus and an ac power frequency while synchronizing the frequencies to each other. Next, the obtained waveform is superposed on an image signal containing the flicker noise. In this manner, the flicker noise contained in the image signal can be eliminated.
Alternatively, a conventional image capture apparatus including a circuit functioning to eliminate a flicker noise is as disclosed in Japanese Patent Application Laid-Open No. 6-038078, for example. In the image capture apparatus disclosed in this reference, a signal level of an image signal actually output from an imaging device (hereinafter also referred to as a “CCD imaging device”) is compared with a reference signal level of an image signal to be obtained by capturing an image using the CCD device in an appropriate (reference) exposure time, to find a difference between an actual exposure time and the reference exposure time. Next, calculation is carried out using two data of: data of the reference exposure time which corresponds to an exposure time of one cycle before a cycle where a flicker noise is caused due to a difference between a vertical scanning cycle actually employed in the image capture apparatus and a variation cycle of change in light intensity of illumination provided to a subject; and data of the difference between the actual exposure time and the reference exposure time. As a result of the calculation, a new exposure time to be set in the CCD imaging device is determined. Further, data of a drive timing is obtained in order to implement the new exposure time to be set. On the other hand, the image capture apparatus is provided with an oscillator for generating a pulse occurring plural times at constant intervals which are counted by a counter. Then, the counted times of the pulse and the data of a drive timing are compared with each other by a comparator, to detect a coincidence point at which any of the counted times of the pulse coincides with any value of the data of a timing for driving. Thereafter, in the image capture apparatus, an information about an optical image of the subject is stored in the CCD imaging device during a period of time from the coincidence point to a point at which a stored charge is read out during a vertical retrace period. In this manner, the conventional image capture apparatus eliminates the flicker noise contained in the output image signal.
In accordance with the above-described conventional method of correcting a flicker noise which is disclosed in the first-cited reference, a waveform having a phase reverse to that of the flicker noise is obtained by synthesizing and synchronizing operations, which requires detection of an ac power frequency. For this reason, there is a need of additionally providing a device for detecting an ac power frequency in an image capture apparatus employing the conventional method. This complicates a structure of the image capture apparatus, as well as increases the size thereof.
On the other hand, in accordance with the above-described conventional image capture apparatus which is disclosed in the second-cited reference, a signal level of an image signal actually output from a CCD imaging device is compared with a reference signal level of an image signal to be obtained by capturing an image using the CCD element in an appropriate (reference) exposure time, to find a difference between an actual exposure time and the reference exposure time. Such procedure together with the other procedures is so complicated as to necessitate an additional process time for eliminating a flicker noise.